Add build action to ignore specific templates

This is useful for cases where a template isn't finished or has been removed from the VSIX but you want to keep it in the project.

Proposed: Ignored.

NOTE: this is not strictly needed, since anyone can modify the .csproj and set the item name to whatever they want, but it's a convenience to essentially get back the "None" behavior for .vstemplates we don't want to deploy.
